This invention relates generally to the field of keratoscopic instruments and more particularly to corneal topographers.
In a number of ophthalmic surgical procedures it is desirable to have a three dimensional or topographic map of the cornea. In particular, corneal topographers can be extremely useful in conjunction with radial keratotomy surgery, phototherapeutic and photorefractive keratectomy procedures as well as in conjunction with contact lens fitting procedures.
Several corneal topographers are commercially available and each operates in basically the same manner. A series of concentric light rings are projected onto the cornea and are reflected back into the topographer, where the reflected ring pattern is captured electronically by a CCD camera and digitized. If the reflecting surface is perfectly spherical, the reflected ring pattern will be identical to the projected ring pattern. If the reflecting surface is aspherical, the reflected ring pattern will be distorted, and this distortion can be translated through a series of algorithms into a three dimension plot of the reflecting surface. The operation of ring pattern corneal topographers is more fully discussed in U.S. Pat. Nos. 4,685,140, 4,772,115, 4,863,260, 4,978,213, 4,995,716, 4,902,123, 5,009,498, 5,018,850, 5,062,702, 5,106,183, 5,159,361, 5,227,818 and 5,300,965 the entire contents of which are incorporated herein by reference. Other corneal topographers using reflected laser beam imaging systems are under development but not yet commercially available.
With ring pattern corneal topographers, the sharpness of the reflected ring pattern directly affects the accuracy of the topographic measurement because the reflected pattern is digitized prior to being electronically processed and an out of focus or fuzzy image may give an inaccurate reading or may not be intense enough to be perceived by the topographer electronics as a data point. Therefore, commercially available corneal topographers all contain a device to either automatically focus the light rings on the cornea or to assist the operator in manually focusing the light rings.
For example, the corneal topographer manufactured by Computed Anatomy and sold by Tomey uses two low powered aiming lasers that produce spots on the cornea. The laser are arranged so that when the spots precisely overlap, the topographer is in focus. Another topographer manufactured and sold by EyeSys Technologies, Inc. uses a cross-hair sight that must be aligned within an illuminated target to ensure proper focus. Still another topographer, the EH-270, manufactured and sold by Alcon Laboratories, Inc., uses an LED and a stepper motor driven focusing system. The stepper motor moves the optical system along the optical axis to five known points where the intensity of the reflection of the LED off of the cornea is measured. Using a curve fitting algorithm, the topographer calculates the theoretical peak intensity and its location along the optical axis and the stepper motor moves the optical system back to this calculated "optimal" focus location. This method is more fully described in copending and commonly assigned U.S. Patent Application Ser. No. 08/046,619, filed Apr. 14, 1993, the entire contents of which is incorporated herein by reference.
While the focusing methods used by the topographers discussed above are accurate, triangulation-based methods such as aiming lasers and cross-hair sights require a certain degree of operator skill, and the LED/stepper motor method requires the use of a relatively precise (and expensive) mechanical positioning system.
Accordingly, a need continues to exist for a simple, inexpensive and precise method for focusing a corneal topographer.